Molded rubber keyboards have been used in combination with underlying printed circuit boards to provide a keyboard structure much less expensive than those where each key requires a separate push button member separately mounted in a frame. The term "rubber", as used in this specification and in the claims, refers not only to synthetic and natural rubbers, but also to other elastomeric materials.
Molded rubber keyboards usually comprise a rubber sheet integral with upwardly projecting rubber keys. Each rubber key is located above a recess in the sheet and its lower end includes such means as conductive rubber members for closing the switch contacts on the PC board located directly below it. The recesses form a diaphragm which acts to space the conductive rubber members above the PC board, except when a particular key is pressed.
In spite of their advantages, these molded keyboards have also had some disadvantages. The principal disadvantage is that when a key is pressed down, it tends to wobble. Although the wobbliness may not actually affect the key's electrical function, it does affect its feel and the attitude of the user toward it. The user tends to view the whole instrument which incorporates the keyboard as lacking precision, and of being of low quality, simply because of the wobbling of the keys. Additionally, the wobbling effect does not inspire confidence that the switch contact has been made.
Attempts have been made to solve this problem. For example, guide pins were sometimes inserted in the center of each key and extended into corresponding holes in the printed circuit board. However, this approach added a number of parts--at least one pin in one hole for each key--and therefore considerably increased the overall cost. Moreover, the holes had to be drilled not only through each key but also through the PC board in a position through the center of its contacts, so that the drilling became a precision operation and was correspondingly expensive.
Another attempted solution was to mold the keys so that they were hinged on one side. However, such keys must then be activated in a rocking motion instead of straight line motion, and the response of many users was that they preferred even wobbly keys over this alternative.
Another proposed solution was to provide a molded or machined sleeve for each key and then insert the key buttons in the sleeves. The sleeves were to be of stiff plastic or metal. The sleeve was to provide a mechanical guide for the key button. However, when the key button was depressed, it tended to stick on the side of the sleeve, holding the switch open. Since they were not integral with the molded keyboard, many additional parts were required; so while there was a gain in the prevention of wobbling, the added expense was undesirable.
Another solution that was proposed was to provide a molded plastic key which would ride in a molded or mechanical sleeve and make contact with the rubber key when the key was depressed. Many additional parts were required; so while wobbliness was prevented, the added expense was undesirable.
An object of the present invention is to solve the problems of wobbliness and binding, while holding the number of extra parts down to a minimum.